The present invention relates generally to microelectromechanical structures (MEMS) and, more particularly, to MEMS variable capacitors and associated operating methods.
Various microelectromechanical structures (MEMS) and other microengineered devices exist for providing voltage or current controlled capacitive elements that may be used, for example, in tunable circuits such as voltage controlled oscillators (VCOs). One such MEMS device is a MEMS variable capacitor. Conventional MEMS variable capacitors may include a pair of capacitor plates, one of which is typically disposed upon and fixed to a substrate and the other of which is typically carried by a movable actuator or driver. For example, one such variable capacitor is disclosed in U.S. Pat. No. 6,229,684 to Cowen et al. entitled Variable Capacitor and Associated Fabrication Method, the disclosure of which is incorporated herein by reference. This application discloses one of the capacitor plates disposed on a cantilever and the other of the capacitor plates disposed upon and fixed to a substrate. The capacitor plate disposed on the cantilever is used to vary the separation of the capacitor plates to provide a voltage controlled capacitor, the capacitance of which depends on the amount of separation of the capacitor plates.
Variable capacitors according to embodiments of the present invention include first and second bimorph members spaced apart from a substrate. A first capacitor plate and a second capacitor plate are mechanically coupled to the first and second bimorph members, respectively. The second capacitor plate is positioned in a spaced apart relationship to the first capacitor plate. An actuator is associated with at least one of the first and second bimorph members and is configured to vary the spaced apart relationship between the first and second capacitor plates. In some embodiments of the present invention the actuator is associated with both the first and the second bimorph members. The actuator may be a thermal actuator, an electrostatic actuator and/or an electromagnetic actuator.
In some embodiments of the present invention, the first bimorph member and second bimorph member comprise first and second layers having dissimilar thermal coefficients of expansion. In further embodiments of the present invention, the first layer comprises silicon and the second layer comprises nickel. A first and a second insulating layer, for example, silicon nitride, is positioned between the first and second layers.
These first and second bimorph members are configured to respond to changes in ambient temperature, i.e. the thermal environment. Therefore, both bimorph members preferably move in tandem in response to changes in ambient temperature, thus maintaining a consistent spaced apart relationship between the first and second plates that are mechanically coupled to the first and second bimorph members. Accordingly, a current may be provided to one or both of the bimorph members causing the capacitor plates to move together or apart and thereby causing a variation in the capacitance between the capacitor plates. This variation can remain constant notwithstanding changes in ambient temperature.
In further embodiments of the present invention, a variable capacitor is provided having first and second capacitor plates, a tandem mover and an actuator. The first and second capacitor plates are positioned such that the first and second capacitor plates face one another in a spaced apart relationship. The tandem mover, for example, a pair of bimorph members, is configured to move the first and second capacitor plates in tandem in response to changes in ambient temperature to maintain a consistent spaced apart relationship between the capacitor plates. The actuator is then configured to vary the consistent spaced apart relationship maintained by the tandem mover in response to an external input. The capacitance of the variable capacitor can therefore be varied by increasing and decreasing the spaced apart relationship between the first and second capacitor plates, while remaining independent of changes in ambient temperature.
Methods of operating a variable capacitor according to embodiments of the present invention include positioning a first capacitor plate and a second capacitor plate so that the first and second capacitor plates face one another in a spaced apart relationship. These methods further include moving the first and second capacitor plates together (in tandem) in response to changes in ambient temperature to maintain a consistent spaced apart relationship between the first and second capacitor plates. At least one of the first and second capacitor plates is actuated to vary the spaced apart relationship between the first and second capacitor plates in response to an external input to thereby vary the capacitance between the capacitor plates.
In some embodiments of the present invention the variable capacitor includes a third bimorph member that is spaced apart from the substrate. A third capacitor plate is mechanically coupled to the third bimorph member and positioned between the first and second capacitor plates. The actuator is associated with the third bimorph member and configured to increase the spaced apart relationship between the third capacitor plate and one of the first and second capacitor plates and decrease the relationship between the third capacitor plate and the other of the first and second capacitor plates.
In further embodiments of the present invention the variable capacitor includes third and fourth bimorph members that are spaced apart from the substrate. A third and a fourth capacitor plate are mechanically coupled to the third and fourth bimorph members, respectively. The first, second, third and fourth capacitor plates are positioned such that the first, second and third capacitor plates face the fourth capacitor plate. The actuator may be associated with at least one of the first, second, third and fourth capacitor plates and configured to vary the spaced apart relationship between the first, second, third and fourth capacitor plates.
In yet further embodiments of the present invention, the substrate may include a trench. The first and second bimorph members deflect into the trench in response to the actuator. The first and second bimorph members also deflect into the trench in response to changes in ambient temperature. In other embodiments of the present invention the first and second bimorph members may deflect away from the substrate in response to the actuator.